Action for gravity, possible alternatives and implications

In the first part of my talk I will discuss the variational principle for general relativity and some associated subtle facts not shared by other interactions present in the nature. In particular, I will try to arrive at the correct action principle for general relativity in presence of null boundaries. Possible generalization to Lovelock theories will also be presented. In the second part, I will talk about theories beyond general relativity, concentrating on higher dimensional models and their possible astrophysical implications. I will mainly concentrate on the effect of extra dimensions on the propagation of gravitational waves and accretion processes around supermassive black holes. Implications and future directions will also be discussed.